Muscle weakness

Muscle weakness
Classification and external resources
ICD-10	M62.8
ICD-9-CM	728.87 (728.9 before 10/01/03)
DiseasesDB	22832
MeSH	D018908
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Muscle weakness or myasthenia (my- from Greek μυο meaning "muscle" + -asthenia ἀσθένεια meaning "weakness") is a lack of muscle strength. The causes are many and can be divided into conditions that have either true or perceived muscle weakness. True muscle weakness is a primary symptom of a variety of skeletal muscle diseases, including muscular dystrophy and inflammatory myopathy. It occurs in neuromuscular junction disorders, such as myasthenia gravis. Muscle weakness can also be caused by low levels of potassium and other electrolytes within muscle cells. It can be temporary or long-lasting (from seconds or minutes to months or years).

Muscle weakness can be classified as either "true" or "perceived" based on its cause.

In some conditions, such as myasthenia gravis muscle strength is normal when resting, but true weakness occurs after the muscle has been subjected to exercise. This is also true for some cases of chronic fatigue syndrome, where objective post-exertion muscle weakness with delayed recovery time has been measured and is a feature of some of the published definitions.

Muscle weakness can also be classified as either "proximal" or "distal" based on the location of the muscles that it affects. Proximal muscle weakness affects muscles closest to the body's midline, while distal muscle weakness affects muscles further out on the limbs.

Proximal muscle weakness can be seen in Cushing's syndrome and hyperthyroidism.

The severity of muscle weakness can be classified into different "grades" based on the following criteria:

Neuromuscular fatigue can be classified as either "central" or "peripheral" depending on its cause. Central muscle fatigue manifests as an overall sense of energy deprivation, while peripheral muscle fatigue manifests as a local, muscle-specific inability to do work.

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